Containers made of metal, glass or a thermoplastic material are commonly used to package various food products such as vegetables and fruit which are normally filled at elevated temperatures. Some food products may require an additional cooking or retorting cycle, after which the package is cooled to room temperature. Using atmospheric pressure as zero pressure, these packaging procedures produce internal pressures during the process cycle and partial vacuums during the cooling cycle. A partial vacuum in a glass container does not produce sidewall buckling or paneling because of the structural integrity of the glass itself. However, in thin-walled metal containers, especially aluminum containers, and even more so in plastic containers, sidewall paneling becomes noticeable for hot filled food products. This sidewall deformation results from the differential pressure present between the interior and exterior walls of the container. The differential pressure is increased whenever the packaged product chemically reacts with oxygen present in the headspace of a filled and sealed container. Such chemical reaction causes a decrease in the internal pressure of a container.
Those skilled in the art know that variations in a product's fill temperature, headspace, product volume, container expansion-contraction properties, processing conditions, and type of product to be packaged all influence the final differential pressure acting between the interior and exterior walls of a container. These packaging variations can lead to different degrees of sidewall buckling or paneling which must be overcome in order to present a commercially acceptable and aesthetically pleasing package. Sidewall paneling can also adversely affect a container during shipping and handling by creating excessive column load and/or prevent proper nesting and stacking with adjacent containers.
Present measures taken to prevent sidewall distortion in thin-walled metal and plastic containers vary depending upon the package design. Some containers utilize sidewall ribbing or beading to provide structural strength while others use bellows or buttons formed in an endwall which flex according to changes in the container's internal pressure or vacuum. Another measure taken to limit sidewall flexing incorporates the injection of liquid nitrogen which expands in the sealed container to preclude the development of a partial vacuum inside the container. Few containers use an increased sidewall thickness because it is not economically feasible.
A specific way of providing sidewall support and decreased internal vacuum is taught in U.S. Pat. No. 3,117,873. In this patent, a mechanical force is applied to an endwall causing a bead formed in the sidewall to collapse thereby shortening and stiffening the container. An alternative procedure, shown in FIGS. 10-14 of the '873 patent, reverses a domed endwall by use of a plunger to accomplish limited volume displacement in an uncontrolled manner. The following U.S. Pat. Nos.: 4,381,061; 4,222,494; 4,177,746; 4,134,354; 3,409,167; 3,400,853 and 3,160,302, teach the use of a flexible endwall on a container which can flex from a convex to a concave configuration in a cricketing or reversing fashion so as to minimize sidewall distortion. Lastly, specific ribbed and paneled sidewall designs are shown in U.S. Pat. Nos. 3,497,855 and 4,120,419.
Despite the above-suggested solutions to the sidewall paneling problem, there is a very real need for an improved and cost-efficient container. This need is most noticeable for containers designed to be filled with a hot product and sealed before being cooled to room temperature and for containers designed to be filled with a product at room temperature before being pressurized. Now a container has been invented which can satisfy this need.